Owing to its transparency and dimensional stability, amorphous resin such as poly(methyl methacrylate) (hereinafter referred to as PMMA) and polycarbonate (hereinafter referred to as PC) is widely used, for example, in the field of various parts of optical materials, electric appliances for household use, OA appliances, automobiles, etc.
Recently, the resin of the type has become used more widely even for optical materials of higher performance, such as optical lenses, prisms, mirrors, optical discs, optical fibers, sheets and films for liquid-crystal display, optical waveguides, etc. With that, the resin is required to have further better optical properties and more improved heat resistance.
At present, the transparent resin is used also for lighter parts for automobiles, such as tail lamps and head lamps, and the recent tendency is toward reducing the distance between various lenses such as tail lamp lenses, inner lenses as well as those in headlamps or sealed beams, and the light source and toward thinning the constitutive parts for increased roominess and for better gasoline efficiency. In addition, since vehicles are driven under severe conditions, the resin for them is required to deform little at high temperature and high humidity and to have good scratch resistance, weather resistance and oil resistance.
Though having good transparency and weather resistance, PMMA resin is problematic in that its heat resistance is not satisfactory. On the other hand, PC resin has good heat resistance and impact resistance, but its birefringence that indicates the optical strain thereof is large. Therefore, the resin is problematic in that its moldings have optical anisotropy and their scratch resistance and oil resistance are extremely bad.
Accordingly, for improving the heat resistance of PMMA, a resin has been developed by introducing thereinto a maleimide monomer, a maleic anhydride monomer or the like that serves as a component of making the resin resistant to heat. However, this is problematic in that the maleimide monomer is expensive and its reactivity is low, and maleic anhydride is not stable to heat.
To solve these problems, JP-A 49-85184 and 1-103612 disclose a copolymer that contains glutaric anhydride units, and this is obtained by heating a copolymer that contains unsaturated carboxylic acid monomer units. However, since the polymerization temperature in producing the unsaturated carboxylic acid monomer units-having polymer is high, the glutaric anhydride units-having copolymer, which is obtained by heating the polymer by the use of an extruder, is still problematic in that it is extremely discolored.
On the other hand, JP-A 58-217501, 60-120707 and 1-279911 disclose a method for producing a glutaric anhydride units-having copolymer by heating a solution of an unsaturated carboxylic acid monomer units-having polymer in vacuum. However, even in the method disclosed in these patent publications, the polymerization temperature in producing the unsaturated carboxylic acid monomer units-having polymer in solution is still high. Therefore, even when the polymer solution is heated as such in vacuum in the method, the effect of retarding the discoloration of the resulting glutaric anhydride units-having copolymer is still unsatisfactory and it does not satisfy the recent requirement for a higher degree of colorlessness. Another problem with the method is that, the resulting copolymer is extremely discolored during the residence at high temperature in the air, or that is, its dwell stability (discoloration resistance) is not good, and therefore it could not be recycled.
JP-A 60-120735, 61-271343 and 9-48818 disclose a method of reducing discoloration through addition of antioxidant such as (hypo)phosphite compounds or hindered phenol compounds. The method may be effective for glutaric anhydride units-having polymer of which the heat resistance is relatively low, but is still problematic in that it could not satisfy both high heat resistance and colorless transparency.
Accordingly, an object of the invention is to provide a thermoplastic polymer which has high-level heat resistance, which solves the problem of discoloration during heating in the related art, which is prevented from being discolored while heated to form glutaric anhydride units in the resulting copolymer, and which has high-level colorless transparency and dwell stability satisfying the recent requirement; and to provide a method for producing it, and moldings that comprise the thermoplastic polymer.